Create pop up message for redirected customers

Hi,

I have rebranded my business and have a new URL with new content. When I do a 301 redirect from my old URL I want a pop up message telling customers they have been redirected to new site (eg: This is our new page, formerly....)

It would be great if it were a one time pop up when someone first lands on the page from the old URL. 

I know it can be done but not sure how?? 
 

Thanks, 

David